Rheometer with axial resistive force measurement

ABSTRACT

A rheometer comprises a vessel mounted on a stand and a rotatable blade mounted above the vessel. A substance for rheometric assessment is put in the vessel. The rotor is advanced linearly into the substance while at the same time rotating. The blade is arranged with a profile such that there is substantially zero slip as it progresses, thereby minimising disturbance to the substance. The rheometric assessment is based on the axial force exerted as a result of the resistance of the substance to the progression of the blade through it.

This invention relates to rheometry and to rheometers for assessingrheometric characteristics of a substance.

Various forms of rheometer are known which depend upon the determinationof torque required to rotate a member through a body of material therheometric characteristics of which are under investigation.

In effect, the torque is a measure of the resistance to the movement ofthe member presented by the substance. Examples of known rheometers arefound in Chapter III ‘Some Commercial Rotational Viscometers’ of thebook ‘A Laboratory Handbook of Rheology’ by Van Wazer et al.,Interscience 1966 which is incorporated herein by reference. Suchdevices are used to gauge characteristics such as, for example,viscosity, flow, homogeneity, etc. The torque required to rotate themember through the substance is related to the resistance to movementpresented by the substance itself. While the book is of a considerableage, it is still the case that its disclosures are broadlyrepresentative of the prior art in the field of rheometry today.Consistent among the teachings of the known devices is the use of arotating member. However, because the characteristic movement of themember is rotary, it is cyclical. In certain circumstances the cyclicalnature of the movement of the member means that it passes through adisturbed region of the substance after the first passage of the member.Thus, unless the substance is one which is able substantially fully torecover before the next disturbance by the member at the rate at whichit is rotating, the torque required to drive the member will bedifferent from cycle to cycle.

The book referred to above discloses a modification of the BrookfieldViscometer on page 144 in which the basic hand-held Brookfieldinstrument is mounted on a ‘Helipath’ stand above a vessel containing asubstance under investigation. The mounting for the device on the standis motor driven so that it can lower the viscometer at the same time asit is rotated. The member describes a helical path as it progresses. Byvirtue of this helical progression, the member encounters onlyundisturbed material as it rotates. As is stated in the book, theHelipath Brookfield Viscometer is used for material with high yieldvalues or extreme thixotropic or rheopectic effects. In short, materialsunable to recover due to the passage of a blade can more accurately beassessed for rheometric characteristics by a member describing a helicalpath through the material. In keeping with the rest of conventionalrheometer practice, the rheometric characteristic is assessed bymonitoring the torque required to rotate the member at a given rate.

A later example of a rheometer is disclosed in EP-A-0798549, which isincorporated herein by reference. In the disclosed device the member isagain both rotated and driven linearly into the substance. Whilemonitoring the axial forces produced is intended as an option, it is forcontrolled compression of the substance or for feedback in controllingthe device. While powders are mentioned, it fails to distinguish betweenthem, and liquids and viscous solids. The rheometric assessment of acharacteristic of the substance only involves monitoring the torque inaccordance with conventional practice. Thus, torque remains therecognised quantity by which rheometric assessment is carried out.

According to the present invention, there is provided a rheometercomprising: a vessel for a substance to be rheometrically assessed; ablade member; drive means for rotating and axially moving the blademember through the substance in the vessel; means for monitoring aparameter indicative of the axial resistive force of the substance tothe passage of the member through the substance; and means for derivinga rheometric assessment of the substance from the monitored parameter.

It has been recognised by the inventors of the present invention that itis possible to derive usable rheometric readings from the measurement ofaxial force generated from a progression of the member through thesubstance, particularly a powder, and to ignore rotational forces. Thisis against decades of previous thinking which was based on deriving suchreadings from rotational forces. Furthermore, the monitoring of axialforce is particularly straightforward, versatile and reliable incomparison with torque. Devices for measuring torque are more expensivethan linear force transducers. Torque sensing devices are more difficultto install, set up and calibrate. Furthermore, torque sensing devicesare less reliable in use and are prone to ingress by contaminants. As inconventional rheometers, the member in the present invention is alsorotatable. However, the rheometric assessment is not based on sensingthe rotary aspect of the motion in the present invention.

By sensing axial force, or a parameter indicative of it, and thedistance travelled, the work done can be assessed. Such axial monitoringcan be carried out in one direction or both. The distance can be a setdistance within the substance or a distance between an upper surface ofthe substance and a preferred lower position.

The member is preferably mounted on a shaft or spindle and may be drivenby one or mote suitable electric motors, such as a stepper motor, toprovide the degree of control and resolution over its axial movementand, when applicable, its rotary motion.

The member is desirably a multi-bladed device, but could be singlebladed, having a blade or blades extending outwardly from the axis ofthe member. It is found that a particularly suitable member has a bladeor blades which have surfaces defining an aspect ratio, and which arearranged, so that leading and trailing edges, in relation to themovement of the member, are defined along the substantially axial path.For example, a propeller form for the blade could be used. This has aparticularly advantageous construction. The speed of progression androtation of the member can be set such that the blade progresses throughthe material with substantially zero slip, thereby minimising thedisturbance to the substance in the vessel.

The means for monitoring may be a load cell, such as a strain gauge,attached to the rheometer in order for the axial force arising from theprogression of the member through the substance to be sensed. The loadcell may be arranged in a support for the vessel, thereby sensing theforce transmitted through a substance to the vessel. Alternatively, theload cell may be arranged in relation to the drive means to sense thereactive force transmitted from the substance through the member.

Other means for monitoring a parameter indicative of the axial force mayderive an indication from the drive means of the work done in moving themember. For example, when the drive means is an electric motor the workdone can be derived from the current by means of a current sensingdevice in the electrical supply to the motor. The movements (e.g.revolutions or part revolutions) of the drive means may be monitored toprovide additional data for analysis.

By monitoring a parameter indicative of axial force it is possible tocarry out a first movement of the blade member through the substanceuntil a predetermined load is reached and then to carry out aTheological assessment, or an additional one, at that point. Rheologicalassessment can be carried out on both the forward and reverse axialmovements of the blade.

The invention also extends to a method of rheometric analysis of asubstance comprising: preparing a substance for analysis in a vessel;rotating and axially driving a blade member through the substance toshear it; monitoring a parameter indicative of the axial resistive forceof the substance to movement of the member through the substance; andderiving a rheometric assessment of the substance from the monitoredparameter.

The present invention can be put into practice in various ways, some ofwhich will now be described by way of example with reference to theaccompanying drawings in which:

FIG. 1 is a general section of a rheometer according to an embodiment ofthe invention;

FIG. 2 is a partial view of the rheometer of FIG. 1;

FIG. 3 is an exploded view of the gantry of the rheometer;

FIG. 4 is an exploded view of a gantry part of the rheometer;

FIG. 5 is an exploded view of the base of the rheometer;

FIG. 6 is detail of a vessel and blade of the rheometer; and

FIG. 7 is a schematic diagram of a system for rheometric assessmentincorporating the rheometer of FIG. 1.

Referring to FIGS. 1 and 2, one embodiment of a rheometer has a frame 10comprising a foot 12, an upright 14 extending from the rear of the foot12 and a gantry 16 extending from the upright over the foot 12. Aplatform part 18 is formed on the foot 12 supporting a test vessel 20 ina collar 22 mounted above the platform 18 on the sensing head of a loadcell (to be described). The vessel in this embodiment has a 50 mminternal diameter and can hold a sample of 140 ml, approximately 70 mmin height. Other vessel sizes and suitably fitting blades can be used.

In FIG. 3, the arm 16 comprises a base plate 24 to which is mounted arotation drive motor 26 having a drive shaft 28 carrying a gear wheel 30and extending up through the base plate 24. A bearing assembly 32 ismounted on top of the plate 24 above the vessel 20. A toothedfabric/rubber timing belt 33 drivingly connects the gear wheel 30attached to the drive shaft 28 of the motor to an input gear 34 of thebearing assembly 32.

A rotor member 36 is secured to the output 38 of the bearing assembly 32by a threaded retaining collar 40. As best seen in FIG. 6, the rotormember 36 has a propeller-type blade 42 at the end of a vertical shaft44 which is positioned with its axis above the centre of the circularvessel 20. The propeller blade can be rotated in other ways. The rubbertiming-type belt 33 is found to provide a particularly smooth andreliable form of transmission.

Referring also to FIG. 4, the gantry 16 is mounted for linear movementabove the vessel 20. A threaded attachment 48 is secured to the gantry16. The attachment 48 is engaged with a thread of a vertical drive rod50 passing through the gantry 46. The rod 50 is mounted in upper andlower bearings 52 in the frame 10 between front and rear verticalsupporting stanchions 54 on which the gantry 46 rides vertically.Beneath a lower bearing 52, the drive rod 50 is fitted with a drivewheel 56. A further timing belt 58 transmits rotation from a furtherstepper motor 60 to the drive wheel 56. As the rod 50 is rotated, thegantry 46 rides linearly along the thread, guided between the verticalstanchions 54.

Referring to FIG. 5, to monitor the axial resistance of a substance tothe movement of the blade 42 through it, a load cell 74 is mounted on abase portion 72 on the platform 18. The load cell 74 extends from theedge of the base into the centre and beneath an upper cover 76 mountedon the base portion 72. A mounting adaptor 70 protrudes through acentral aperture in the cover 76, supporting the collar 22 holding thevessel 20 (see FIG. 1). The collar 22 is bolted to the mounting adaptor70 to keep it in place. The load cell is bolted to the base 72. In thisarrangement the load cell senses only axial forces exerted on it fromthe vessel 20. The output (not shown) of the load cell 74 is anelectrical signal which is connected to electronic circuitry (not shown)in the foot of the apparatus. The electronics conditions the output ofthe load cell for subsequent output to a processing device, such as apersonal computer (PC). This is described in further detail below. Thedata is analysed by the PC as necessary to provide quantitativerheometric results based on the force sensed by the load cell as aresult of the progression of the rotor member 36 through the substancein the vessel. The load cell in this embodiment is a wire straingauge-based device well known to the skilled person. Other devices couldbe used, such as a semiconductor-based strain gauge or a linear variabledifferential transformer (LVDT), to sense the axial force.

FIG. 7 shows a rheometer system comprising a rheometer 100 as describedand an analogue-to-digital converter 102 which is supplied with theanalogue output from the load cell 74. The digitised load cellinformation from the converter 102 is fed to a processor 104 which isconventional in construction and arrangement, having a memory 106 and anoutput connected to a plotter 108. An input/output user interface 110 isprovided in the form of a keyboard/monitor or PC. Data captured in testscarried out by the system can be displayed on the monitor in numericalor graphical form according to known techniques. The processor 104 isprogrammed to control the motors driving the blade linearly andangularly, and to capture data at specified periods over specific pointsin the test cycle. For control purposes based on the axial force appliedfeedback according to conventional techniques can be derived from theoutput of the analog-to-digital connector 102. Motor parameter feedbackand control signals to the motors for axial and angular movements of theblade are transmitted on lines 112 and 114, respectively.

The apparatus can be operated in various ways and the electronics, bywhich the motors are also controlled, is programmed automatically toeffect a sequence of operations according to the manner in which it isrequired to be operated. The programming is conventional. The novelsequence of actions for rheometric assessment are described below.

The primary operation is to arrange the blade 42 above the vesselcontaining the substance to be analysed and for it to be lowered andraised in the substance at a predetermined rate by actuation of themotor 50 and at a rate of rotation caused by actuation of the motor 26,under the pre-programmed control of the processor 104.

The blade 42 on the rotor member has a propeller profile that increasesin pitch angle with respect to the axis of rotation of the blade withincreasing radial distance from the axis. Such a profile has a ‘constantlead’ so that it can effect as clean an entry into the substance aspossible. Such profiles for propellers, by which the shearing of thesubstance is caused and turbulence is minimised, are well-known to theskilled person. The travel of a propeller blade following such a helicalpath is thus made up of simultaneous axial and radial progressions.These rates of axial progress and rotation can be such that the rotorblade enters and progresses through the substance at a rate of zeroslip, but the blade may also be moved such that it progresses at anaxial and radial rate that is greater or less than the helix anglepossessed by the blade by virtue of its overall pitch. Under theseconditions a variable axial force is imparted to the substance. Inaddition any such helical path may be progressed either downwards orupwards. This axial resistance to the passage of the blade followingsuch a helical path is sensed as a strain on the load cell from whichreadings relating to viscosity in a liquid or, more generally flowcharacteristics, can be derived.

Once introduced into the substance in the vessel the test can take placeat any desired depth. The blade is advanced to the appropriate position.The test itself consists of combinations of simultaneous or separateangular and axial movements that may combine to be a helical path ornot. The reaction to these movements is the axial force that is sensed.The Theological or flow properties can, thus, be assessed. The degree offlow is related to the inverse of the force sensed. Varying the ‘slip’(i.e. the difference between rate of axial progress of a given propellertype blade at a given rate of rotation) can be used in the assessment ofthe Theological properties of the substance. The controlling processor104 can be programmed to carry out the desired functions in conventionalmanner.

The apparatus is of use in determining readings of viscosity in liquidsand thixotropic materials. However, the invention is also of particularuse in the pharmaceutical and other industries in which the rheometric(for example, flow) characteristics of powders have to be assessed.

The apparatus may be programmed so that the blade is rotated in theopposite direction to the helix, determined by the blade pitch. Thisalternative helix may be in the equal and opposite sense to the designhelix of the blade pitch. The action of progressing such a helix wouldbe to compress the substance as it travels into the vessel or lift up(aerate) the substance as the blade travels upwards out of the vessel.The reaction to these movements is also to produce axial force, whichcan be measured by a suitable transducer. This data can also be used todetermine the Theological properties of the substance. FurtherTheological properties of the substance can be investigated by rotatingthe blade more quickly or more slowly, to realise a situation ofnegative or positive slip, about this opposite helix.

It has been found useful to describe the blade movements in terms of avector, where the angle of this vector starts at 0°, which woulddescribe a clockwise motion but no axial movement and 90°, woulddescribe axial movement but no rotation. All the angles between thesetwo values would include both movements, the rotational movement beingin proportion to the cosine of the angle and the axial movementproportional to the sine of the angle. Angles beyond 90°, and up to180°, would follow the same rules but the rotation would be in theanticlockwise direction. This method is well known in mathematics. Thedirection of the axial movement is stated as being upwards or downwardsin all cases. Thus the full range of possible movements can be describedsimply by stating an angle in degrees, an axial direction and lastly thedesired speed of travel, or magnitude of the vector in terms ofmillimetres per second. All these parameters refer to the tip of theblade, the processor is informed as to which blade is fitted and isprogrammed to determine the relevant rotational and axial parametersrequired for control by virtue of its programming.

Linear speed and rotational speed are not input by the user at thekeyboard 110. Instead, the user is able to specify the blade type, bladetip speed and angle from which the processor 104 then calculates thelinear and rotational speeds.

In all that follows a blade with a left-handed helix, or in more simpleterms, thread, is used. Where angles and speeds are mentioned, these aremerely to illustrate a method and may in practice vary widely from thesevalues, as indeed may the movements and the order of movements to suitthe requirements of the experimenter or the substance being tested.

Although not shown in the drawings, the blade may usefully have aprofiled boss at its centre further to reduce the disturbance caused tothe substance as the blade passes through it.

When the blade is moving downwards into the base of the vessel,compaction of the powder below the blade occurs when the blade isrotating in a clockwise direction. Compaction occurs until the specifiedtest progression angle reaches 90° at which the blade ceases to rotate,beyond 90° the blade begins to rotate anticlockwise and the blade actionis generally said to be ‘slicing’ including the special case of movingthrough the substance with zero slip. Maximum compaction of a powdershould occur when the face of the blade is impacting the powder normalto its progression angle, in the case of a rotor with a 135° angle thisoccurs at a progression angle of 45° in the downwards direction.

When the blade is moving upwards within the vessel, lift displacement,or aeration, of the powder below the blade occurs when the blade path ismoving in an anticlockwise direction. Aeration occurs at angles greaterthan 90°, at angles below this the blade is said to be slicing. Thisrotational and vertical motion of the blade is seen to lift the sampleon the face of the blade. As upward movement continues through thesampled column, the sample falls over the blade, hence providingaeration. With many powders this displacement mode aerates successivepowder test samples in a uniform manner, removing operator dependence,for satisfactory sampling.

The rotational direction is chosen according to the required testingeffect on the sample and is specified by the angle. If a clockwiserotation is required an angle between 1° and 90° is specified. If ananticlockwise rotation is required an angle between 90° and 180° isspecified.

Powders often have characteristics which are significantly differentfrom both liquids and viscous solids to which the majority of rheometricassessment techniques have been applied, and from which those techniqueshave all been developed. However, for powders it is found that otherconsiderations apply, such as caking and interparticle friction. Cakingis the tendency of a powder to aglomerate or form lumps (cakes) duringstorage or transportation. Interparticle friction, powder cohesion andflow stability are all parameters used to describe the properties of apowder under the conditions of flow. Interparticle friction may changeas a powder flow rate changes. Thus, information on interparticlefriction, or information indicative of it, is very useful in processmanagement in a production facility, quality control and materialdevelopment.

By these movements described above the substance can be disturbed onpurpose to agitate (aerate or mix) it in the vessel prior to arheometric reading being made. The purpose of which would be to ensurethat prior to the start of any test the substance would be in a knownand repeatable state and not dependant on the manner in which anoperator filled the vessel. Such pre-conditioning by compressing oraerating the substance is particularly useful in powder applications in,for example, the pharmaceutical industry.

The apparatus can be pre-programmed to condition the powder column inthe vessel 20. This may be done by rotating the blade 42 in an anticlockwise direction but with only a small amount of axial speed. Thedownwards action is performed at a tip speed of 50 mm/s and an angle of175°, and upwards action is performed at a tip speed of 50 mm/s and anangle of 178°. This movement is usually performed twice over the entireheight of the powder column, which in practice has been found enough topre-condition any powder column. The action performed by these movementslifts the powder slightly throughout the column aerating the powder,whilst at the same time breaking up any agglomerations and then allowingthe powder to fall gently over the top edge of the blade and come torest behind it.

A succession of upward and downward passes of the blade (for example twodown and two up) according to a conditioning phase of a test creates auniform packing density in the powder, i.e. it is free from distortionswhich may otherwise be present due, for example, to the methodof-filling the vessel 20 with the powder under test, or settlement ofthe powder prior to testing. While the initial conditioning movement ofthe blade is usually downwards, some tests may require the conditioningto start in the opposition direction from within the powder, therebyrequiring a reversal of the sequence.

As referred to above, caking is the rheometric property of a powder thatis often of interest. The conditioning in which the blade is reversedinto the powder can be used to compact it into the base of the vessel toa predetermined force value. Then the blade is programmed to moveforwards, slicing up through the compacted material with zero slip,causing minimal disturbance to the sample in the process. This sequenceof compacting and slicing up through the material can be repeated anumber of times. The sequence progressively compacts the powder in thebase of the vessel to homogenise it.

It is often of commercial interest to analyse the data in a way whichgives information on the rate at which the cake is formed. This ismonitored by sensing the axial force on the vessel as compaction takesplace in each cycle. At a given value of sensed force, corresponding toa desired level of compaction, the blade is then programmed to slicethrough the material to provide a reading of the resistance of thematerial to axial movement of the blade. This output is indicative ofthe cohesion of the powder in the compacted state.

The tests are carried out according to a macro written for theprocessor. These are selectable by the user through the keyboard. Thefirst is indicative of the rate at which the cake is formed. In allcases speeds, angles, directions, both upwards and downwards, targetforces and target distances can be varied by the user.

After the pre-test conditioning described above, the top surface of thecolumn of powder in the vessel may be uneven. To address this the rotoris moved according to its compacting motion (i.e. clockwise for theembodiment in FIGS. 1 and 2) towards the surface of the sample at a tipspeed of 20 mm/s and at a rate of rotation equivalent to an entry angleof 2° for the blade. This gradual and gentle progression towards thepowder smoothes the top of the sample. When the strain gauge transmits asignal indicative of a 5 g force exerted on the sample by the blade, thesmoothing process is stopped. This procedure also allows the system torecord the column height during the caking test. This gives data on theextent to which the powder settles in storage.

When the force exerted reaches the target value of 5 g as above, sampletesting continues with the recording of data as the blade moves downthrough the powder at a tip speed of 20 mm/s and an angle of progressionof 20°. The compaction which takes place by virtue of this movement ofthe blade is programmed to stop when the strain gauge signal indicates atarget force on the sample of 1 kg. At this point the rotor returns upthrough the sample at a speed of 10 mm/s and an angle of progression of45°. This angle equates to a rate of progression giving zero slip, asthe blade angle is also 45°, an angle of 45° in the upward directionbeing equivalent to one of 135° in the downward direction. This meansthat the blade cuts through the powder like a knife, creating minimaldisturbance as it does so. This is repeated for five compactions andfour returns. At the end of the fifth compaction, the blade isprogrammed to slice through the compacted powder of the sample at anangle of progression of 175° in sympathy with the blade pitch. As theblade moves, the axial force required to do so is recorded from thebeginning of movement to the end. Finally, the blade is returned back upthrough the sample and out of it and a tip speed of 100 mm/s and anangle of 175°.

From these test movements the macro records:

-   -   the height of the column at the start of each compaction        cycle—as determined by the blade exerting 5 g of force on the        sample    -   the distance travelled when the ‘end force’ of 1 kg is        reached—this is the cake height for each cycle    -   the mean force and work carried out (g.mm) to slice through the        caked (compacted) sample after the five compaction cycles.

The data is applied to a spread sheet in which volume and cake heightratios are set out against material column height. Similarly, the cakestrength is set out in the spreadsheet as mean axial force and the workdone (the area under the force/distance graph for the travel of theblade through the sample).

Another form of analysis is aimed at measuring several parameters duringone test sequence, these are; the interparticle friction and its changeas flow rate changes, powder cohesion and flow stability. This latter isessentially an assessment of change in flow characteristics due, forexample, to attrition effects. These properties are assessed during thepowder flow speed dependent test. This information is important for:

-   -   process modification—if a product manager needs to increase        production rates, it is necessary to know how the powder which        is a constituent of the production process will behave at a        changed level of throughput    -   quality control—manufacturers and customers need to analyse        samples for batch consistency    -   process monitoring    -   ingredient modification

The test for powder flow speed dependence comprises a sequence of twoconditioning cycles to induce homogeneity in the sample. The rotor ismoved down through the sample at a speed of 50 mm/s at an angle of 175°.Each cycle is then completed with an upward movement through the sampleat a speed of 50 mm/s and an angle of 178°.

The testing phase comprises a number at sets of differing speeds, of 2cycles each. The first set of two cycles is as follows:

The rotor is programmed to move down through the powder column at a tipspeed of 10 mm/s and an angle of 5°, compacting the powder in thecolumn. While doing this, the system is capturing data on force, axialdistance, and time. This data corresponds to the resistance of thepowder to being pushed at a controlled flow rate, i.e. the interparticlefriction of the powder. At the bottom of the powder column the rotor isprogrammed to slice through the powder at near zero axial speed and notto measure data, this serves the purpose of stopping any hard compactedlayer building up. The rotor is then programmed to move up through thepowder at a tip speed of 50 mm/s and at an angle of 178°. Again, thedata is being recorded. This data corresponds to an indication of thecohesion of the powder.

Once these two cycles are complete, the next two cycles commenceimmediately in the same format, but with a downward compaction bladespeed of 20 mm/s. At the end of these two cycles the compaction speed ischanged to 50 mm/s, then a further two cycles at a compaction speed of100 mm/s, followed finally by two cycles at 10 mm/s. Data on force,distance and time is recorded as before.

Using the macro on the system, the positive and negative areas of forceand distance for the down and up strokes of the blade through thesample, respectively are recorded and inserted into a resultspreadsheet. The spreadsheet is set up to average the two areas for thecompaction data (as the blade moves down through the powder column) ateach rate of blade travel. These are recorded as the compactioncoefficient at 10, 20, 50 and 100 mm/s. The cohesion (as the rotor movesup through the powder and lifts and separates the compacted powder) isalso recorded from the first two cycles and averaged. The compactioncoefficient for the final two cycles at 10 mm/s is averaged and theratio for that average with the average from the initial two cycles at10 mm/s is derived to assess whether the powder has broken down duringthe testing. This is shown in the results spreadsheet as Flow Stability.A figure of flow stability close to 1.00 means it has not changed at allduring the testing. If the figure is greater than 1.00, it is anindication that the sample has changed during testing (giving a highercompaction coefficient). If it is less than 1.00 the sample has changedto give a lower compaction coefficient.

If the flow stability is close to 1.00 and the compaction coefficient isincreased at higher flow rates, the product is more resistant to flow athigher flow rates (and may result in underfilling in a productionenvironment). If the flow stability is close to 1.00 and the compactioncoefficient is decreasing at higher flow rates, than this shows that theproduct is less resistant to flow (it flows more easily) at higher flowrates. This can be used to avoid overfilling in a productionenvironment. If the flow stability is different to 1.00 then it showsthat the product may be liable to attrition during processing ortransportation which can be investigated further using a textureanalyser to carry out a compaction test on the powder or granules andassess the force to fracture the product, and also to test itselasticity characteristics.

In an alternative test, the substance may be compacted to a specifiedsubstantially constant strain. The machine has the ability to measurethe height of the column prior to compaction (as described above). Thusit is able to then determine how far into the vessel the rotor must beinserted to create a specified degree of strain. Strain in this contextis defined as ‘change of length’ divided by ‘original length’ of thecolumn of substance in the vessel. The rotary motion required tomaintain the helical progression under these circumstances may bedetermined by the user or not used at all. Data collection during thisoperational mode is optional. This mode of operation could be terminatedby the elapse of a specified time or a force reading being exceeded.

As a further alternative, the substance may be compacted (or aerated)under conditions of constant force. In this mode a maximum speed may bespecified at which the rotor will penetrate the substance. When aspecified force is reached, instead of stopping or performing anotheraction/movement, the rotor continues to move but at a new ratedetermined solely by the requirement to continue to exert the specifiedforce upon the substance. The control of the rate is by conventionalfeedback of the strain gange output to the motor control. The rotarymotion required to maintain the helical progression under thesecircumstances may be used or not as determined by the user. Datacollection during this operational mode is optional. This mode ofoperation could be terminated by the elapse of time or distancetravelled.

By limiting the device to the sensing of axial forces the implementationof a rheometer is simplified. Because rheometers are often used incontaminating environments in which the ingress of dust and/or moisture,etc. is a problem, avoiding having to sense the force due to relativerotation of parts is of advantage. Furthermore, devices used to senselinear forces are more easily set up and more reliable and versatilethan devices used to sense torque.

The comprehensive software provided with the powder flow analyser of thepresent invention allows almost any feature of a data characteristic tobe recognised and recorded, including peaks, intermediate thresholdpeaks, gradients, elapsed time and distance events, smooth and jaggedline comparisons, and many other features. In addition analytical macrosfor the data analysis can be written to suit a particular test event oramplify a special characteristic of the samples. The data gatheredincludes the axial distance travelled, time elapsed and axial forceexerted. Generally, for a particular powder the force/work done bothdownward (compaction) and upwards (aeration) provide highly repeatablemeasurements. For these to be made the subject of comparison betweensamples, the starting height (or volume) of the samples within the testvessels must be the same after initial conditioning by the analyser.

It will be appreciated that the data gathered by the rotor with theblade traversing the sample vessel contents is analysed by computing theaxial work done (axial force multiplied by the axial distance travelled)for a number of different fill levels of the sample tube. This data isgathered separately for the upward and downward movements and thenplotted on a graph as work done on the Y axis versus sample volume onthe X axis. The relationship demonstrated by this technique shows thatthe work done increases with increasing sample volume according to anexponential function. However, the exponential magnitude of this curvefunction is so slight that it can be considered linear for the fillvolumes in use at present (i.e. between 120 and 180 ml for a 50 mminternal diameter vessel). This finding is important since it makescomparisons between samples of the same product by using differentvolumes possible. In the pharmaceutical industry there are sometimesonly very small quantities available and, in addition to this, samplesmay be prepared in several locations by different operators.

The measurement of axial force alone in rheometric assessment is uniqueto the present invention. Historically, rheometers have assessed torqueas the associated rotational movement can be carried out indefinitely,not relying on the length of a test bed or depth of vessel, nor anyother physical limitation. The mechanical measurement method as used inthe past required measurement of the forces in steady state due to therepeated nature of the rotation cycle. However, according to theinvention the movement of the blade in the helical form, rotated at agiven speed, imparts thrust to the sample. Therefore, the ability of theproduct to ‘flow’ can be measured by simply measuring the axial force.The ‘flow’ of any one product type will also depend on the density, andin the case of a powder, the packing density. It is not necessary tomeasure the input torque. This will vary with product type but is not assensitive a measure as axial force. By moving the rotor axiallysimultaneous with the rotor linear motion, the action of the rotor canbe a compressive or a lifting one. When the actions are used inconjunction with a selectable data capture system, a powerful analyticaltool is provided.

It will be apparent to the person of ordinary skill in the art thatvarious modifications and variations can be made to the presentinvention. For example, a toothed timing belt is illustrated in thespecific embodiment, whereas the motor may be arranged to drive therotor member in rotation directly or through other forms oftransmission. The rotor member may also be moved linearly other than bya helical screw, such as by belts, or the use of a direct drive from alinear motor. The relative movement between the blade and vessel couldequally well be achieved by motion of the vessel. Additionally, whilethe load cell is arranged to sense forces transmitted through thevessel, it could equally well be arranged to sense forces transmittedthrough the rotor member along the shaft above the vessel. Similarly,readings of the resistance to the movement of the rotor member throughthe substance can be derived from the power consumed by the motordriving the member linearly. For example, current supplied to the motorcould be monitored. The blade is described as being of the propellertype in profile so that it can be used in a way in which the leastdisturbance is created as the substance is sheared by the blade'sleading edge. However, coarser blade or non-propeller-type profilescould be used. Thus, the skilled person will appreciate that variationof the disclosed arrangements are possible without departing from theinvention. Accordingly, the above description of an embodiment and itsvariants is made by way of example and not for the purposes oflimitation. The present invention is intended to be limited only by thespirit and scope of the following claims.

1. A rheometer comprising: a vessel for a substance to be rheometricallyassessed; a blade member; drive means for rotating and axially movingthe blade member through the substance in the vessel; means formonitoring a parameter indicative of the axial resistive force alone ofthe substance to the passage of the member through the substance; andmeans for deriving a rheometric assessment of the substance from themonitored parameter.
 2. A rheometer as claimed in claim 1 in which theparameter is monitored in opposite directions of passage of the memberthrough the substance.
 3. A rheometer as claimed in claim 1 in which themeans for forming include means for calculating the work done in axiallymoving the member.
 4. A rheometer as claimed in claim 3, including meansfor measuring axial distance travelled by the member while the saidparameter is monitored.
 5. A rheometer as claimed in claim 1 in whichthe means for monitoring include means for sensing the force exertedresulting from resistance of the substance to movement of the member. 6.A rheometer as claimed in claim 5 in which the means for sensing theforce include a load cell arranged to sense the axial force transmittedthrough the vessel.
 7. A rheometer as claimed in claim 6, including abase on which the vessel is mounted, and in relation to which the loadcell is mounted to sense the axial force transmitted through the vessel.8. A rheometer as claimed in claim 3, including a stand supporting theblade member, the means for sensing the force being arranged in relationto the stand to sense the axial force transmitted through the member. 9.A rheometer as claimed in claim 1 in which the drive means include anelectric motor arranged to drive one of the blade member and the vesselto achieve relative motion between them the means for monitoring includecurrent sensing means arranged to monitor the current drawn by themotor.
 10. A rheometer as claimed in claim 1 in which the blade membercomprises a shaft having at least one radially extending blade arrangedtowards one end.
 11. A rheometer as claimed in claim 10 in which the oreach blade has a propeller profile with increasing pitch angle withrespect to the axial direction with increasing radial distance from theshaft.
 12. A rheometer as claimed in claim 1 1 in which the drive meansare operable to drive the blade for substantially zero slip.
 13. Arheometer as claimed in claim 12 in which the drive means are operableto drive the blade in the opposite direction to that dictated by theblade pitch.
 14. A method of rheometric analysis of a substancecomprising: preparing a substance for analysis in a vessel; rotating andaxially driving a blade member through the substance to shear it;monitoring a parameter indicative of the axial resistive force alone ofthe substance to movement of the member through the substance; andderiving a rheometric assessment of the substance from the monitoredparameter.
 15. A method as claimed in claim 14 in which the parameter ismonitored in opposite directions of passage of the member through thesubstance.
 16. A method as claimed in claim 14 including calculating thework done in axially moving the member.
 17. A method as claimed in claim16 including measuring axial distance travelled by the member while thesaid parameter is monitored.
 18. A method as claimed in claim 14 inwhich the monitoring includes sensing the axial force exerted by theresistance of the substance to the passage of the member.
 19. A methodas claimed in claim 14 in which an electric motor is arranged to movethe member relative to the vessel, the monitoring including sensing thecurrent drawn by the motor as the member passes through the substance.20. A method as claimed in claim 14 in which the member has at least oneblade extending from an axis and having a profile with an increasingpitch angle with respect to the axial direction with increasing radialdistance from the axis.
 21. A method as claimed in claim 20 includingdriving the blade in the opposite direction to that dictated by theblade pitch.
 22. A method as claimed in claim 20 in which the blade isadapted to pass through the substance with substantially zero slip atgiven axial and rotary speeds.
 23. A method as claimed in claim 14 inwhich the Theological assessment is derived from the monitored parameteronce a predetermined force is reached.